We signed Matri in 2011 (not counting the half year loan) for 15.5m eur, with the original contract expiring in 2015, prolonged later to 2017.
This means that every year of Matri in Juve is worth 2.58m eur. Since he already spent two years here, which takes away 5.16m eur off of his 15.5m eur price, it means that we'll suffer losses if we sell him for less than 10.3m eur.
That's why Marotta doesn't want to go below this price, as mentioned above.